Traction sheaves are used in a variety of load-lifting applications such as elevated platforms for building maintenance. Such a platform can be used in conjunction with a motorized hoisting device, whereby the hoisting device is attached to the platform of an elevated platform or basket that may then be raised or lowered using the hoisting device. The hoisting device and associated rigging typically comprises a traction sheave, whereby the load of a continuous cable or rope passes through the sheave.
The traction sheave is generally designed to operate with significant tension in the cable or rope encircling the sheave. One characteristic of traction sheaves and in particular the single sheave “V” groove type is that the cable or rope's points of contact with the sheave may have different radii at different positions around the sheave. As a result, the velocity of the wire rope may vary according to the instantaneous radius at which each point is in contact with the sheave. Consequently, the wire rope will tend to be either tensioned or bunched as adjacent portions of the wire rope are at lesser or greater radii than the average radius around the traction sheave.
At the points where the traction sheave contacts the cable or rope, friction creates driving tension in the cable or rope. Generally the wire rope will tend to bunch in a known region of the sheave. When sufficient wire rope has bunched and laterally lifted off the tension sheave, the friction between the sheave and wire rope in that region decreases to the point where the tensioned cable or rope may slip or develop slack. The cable or rope may then tighten under the applied load and suddenly resume full contact and friction with the sheave. This event may produce a loud and objectionable popping impulse noise generated when the cable or rope snaps back against the traction sheave. Generally, the tendency to generate the popping noise increases with a poorly maintained dry, non-lubricated cable or rope as the incidence of the bunching may increase.
Several methods of restraining wire rope movement relative to the traction sheave on traction hoists are known. For example, U.S. Pat. No. 4,681,301 discloses a series of rollers restraining the wire rope to the proximity of the sheave groove. U.S. Pat. No. 5,082,248 discloses a segment of rollers restraining the wire rope to the proximity of the sheave groove. U.S. Pat. No. 4,706,940 discloses a pair of rollers restraining the wire rope to the proximity of the sheave groove. A disadvantage of the above disclosed methods is the generally increased cost and complexity in implementing the disclosed devices, and the potential for an increased incidence of rope jams.
Other methods of restraining wire rope displacement relative to the traction sheave have been developed such as that disclosed in U.S. Pat. No. 4,193,311. However, the method disclosed performs a function different to the situation described above. What is needed is a restraint assembly that can reduce or eliminate the lateral rope or cable movement that can result in the objectionable popping noise, and can further be easily removed and replaced without excessive disassembly of the hoisting device and traction sheave and the removal of the lines, ropes or cables.